Why Do Immunizations Require Two Doses

Multiple Choice

Why do immunizations require two doses?

A) The first dose stimulates the effector cells only, and the second dose stimulates the memory cells only.
B) The first dose stimulates the primary immune response, and the second dose stimulates the secondary immune response.
C) In case the first dose is not enough to induce immunity, a second dose is given.
D) The first dose often is ineffective.
E) The first dose stimulates the innate immune response, and the second dose stimulates the adaptive immune response.